Send nested JSON with server-side API

How do I submit a JSON with the server-side API action like this:

{
    "purchase-order": {
        "status": "OPEN",
        "warehouse": "c955_330498075",
        "variants": {
            "vendor": "najel",
            "replenishment": 1
        }
    }
}

Maybe it has something to do with the ‘add repeat’ action, but I’m not sure how to make that work.

Sending a ‘flat’ JSON is working fine.